I just found this forum featuring a lot of Acoustat information and a dependable knowledge base. These threads are a "must read" for any owners of the many versions of this terrific speaker!
I've enjoyed my 2+2's since purchasing them used in the early 80's. Original configuration, non-medallion but label proudly states "Wired with Monster cable".
Year in, year out, never a problem. This is simply a superbly designed, conservatively manufactured piece of first class audio equipment. Everything I have done as a longtime, hardcore dedicated Hi-Fi hobbyist is to adequately feed these speakers.
With that in mind, allow me to add a tiny contributation, that of opinion and experience.
Do perform the "C Mod", regardless of possessing "Medallion" transformers. First, replace all of the high voltage disc caps on the voltage multiplier board.
I see that some had questions pertaining to the physical mounting of the other components. The following pictures demonstrate the method I utilized to mount the capacitor bank (Solon, Mundorf, and Russian Teflon) on two, quarter inch thick phenolic boards.
Mounting screws were added to the top and painted black to camouflage them. The only visible alteration to these beautiful speakers is the Vampire brand speaker binding posts replacing the original, pitiful binding posts. These were never up to the quality level of this classic.
This Photo shows the mounting of the added resistance and wiring changes utilizing Teflon insulated silver and (I think) DB brand Teflon insulated copper.
The Mills resistors, like the capacitor bank are paralleled, with each component measured and matched to achieve the most accurate left/right identity. Many will consider this useless or overkill but this is my hobby. It may seem senseless on a production run of 10000 units but this is the only pair I'm doing and to ignore this minute detail would, to me, be unthinkable.
The final result was neither small nor imagined. I will not go into the magical, musical changes that I heard. Be it the circuitry alterations or the replacement of very old electrolytic capacitors with new film types ...
Perform this Modification!!!

I agree that the C-Mod is well-worth doing for any Acoustat MK-121 series speaker, regardless of whether you have Medallion transformers or not. The changes are independent of each other, and the C-Mod is cheap and easy to do! And a nice job done on this pair!

I own Acoustat 1+1 since 10 years and I love the sound and design.
But I'm not satisfied with the overall stereo image and sound stage.
As comparison I owe old magnepan 3 in an active configuration, which have less transparency and dynamics, but give a lot larger and deeper stereo stage.

In the upper frequency the sound of the Acoustat comes 'from the speaker' and not from behind them as with the Magnepan. It makes the listening a bit tiring. I would call this 'too much directivity in the upper mids'

The curves herejoined represent the frequency response of both speakers in similar conditions.The 1+1 are blue and magnepan are green. Only the impulse response shows a significant difference : 1+1 shows larger dynamics, the facts are very impressive...

I do think that there is something wrong somewhere ...
Here are also a photo of the interface.
Since this photo, I changed the electrolytic cap, the yellow caps, and the high tension ladder diodes and caps. It improved the transparency of the sound, but the stereo remains focused on the speakers.

Can you confirm to me the values of the high pass filter caps in the MK121 interface ?
Is there something I should do to improve things ?
Many thanks,
Mathieu

First off, a special thanks to "AcoustatAnswerMan. Not only were you directly involved in the creation of this audio classic but your participation here will be responsible for its extended life!
Acoustannoy, I followed the "C Mod" pretty closely. The bank of capacitors shown is composed of:
(1) A Solon 47mfd. Film.
(2) A Solon 10mfd. Film.
(3) A Mundorf 1mfd, Film.
(4) A Mundorf .1mfd. and
(5) A .01mfd. Russian Teflon
Please keep in mind that there is resistance added along with a slight (but significant) circuit modification.
I'm hesitant to say this but I've never heard a Magnepan setup that I would prefer over these 'Stats.
It could be speaker location, supporting electronics, room acoustics, or just personal preference but these were very good before the update and magnitudes better afterward.
I highly recommend these changes!!

Factory was closed today, so I do not work, and could spend the whole day updating the interfaces, measuring and listening.

My goal is quite precise concerning the Acoustats : I owe them since more than 10 years. I love the sound, the dynamic, the transparency.
But I want to open their sound and to limit their directivity, which make them a little bit tiring and frustrating sometimes.

The model I have had already the medallion and Cmod performed. I just changed all the components for new and better ones, and cut the low frequencies thanks to a 12db/oct High pass passive filter between the preamp and power amp. An active sub is doing the job under 50Hz.

But,
I had the most impressive improvement today, through the addition of the cap as recommended on
Izzy Wizzy AudioIzzy Wizzy Audio
(sketch herewith - the 0,01microfarad 5000V is a yelow polyester cap)

Modification is easy to do, and for the first time I had a significant improvement of the stereo image in the two directions : it is broader and deeper, the sound does not stick to the speakers as before ! Female voices are impressive.

1- I highly recommend this mod

2- I don't understand the technical reason

3- what can be done to go even further ? Does increasing the cap value makes sense ?